Carpooling with three or more people can lower the climate impact of a car ride to that of a train ride, according to an analysis in Environmental Science & Technology.

The researchers calculated the climate-related impact of travelling 500 to 1,000 kilometers by car, train, coach, or plane. The team accounted not only for carbon dioxide emissions but also aerosols, chemicals that contribute to ozone formation, and in the case of airplanes, the effects on clouds. As one might expect, flying generally had a higher impact than gasoline cars, followed by diesel cars, trains, and diesel coaches.

But the results got more interesting when the researchers tested different occupancy levels. If a person drove a car alone, the climate impact per person could be as high as that of a typical airplane flight. But if a car held at least three people, its climate impact per person was on par with that of a sparsely-occupied train or coach. Small cars that ran on diesel performed particularly well, the team says. — Roberta Kwok | 2 July 2013